IR dichroism, measurements

The IR dichroism measurements allowed a fairly precise determination of the preferential molecular conformations both in the smectic Ai and X phases (see Sect. 2.3). In the smectic Ai phase the biphenyl moiety is parallel on average to the layer normal, while the hydrocarbon and perfluorinated fragments are tilted at angles 18 and 32°, respectively. The phase transition to the smectic X phase is accompanied by a dramatic change in the main molecular conformation - now all the fragments are strongly tilted with respect to the layer normal (especially the biphenyl core which tilts at an angle of around 56°) (Fig. 12). 

The detailed information about molecular orientation in oriented film specimens can be obtained from IR dichroism measurements (Hu et al., 1985 Wu et al., 1986). Light is absorbed when the direction of oscillation of its electrical vector has a component along the direction of oscillation of the absorbing group. The intensity of the absorption band of an oriented polymer thus depends on the direction of the electrical vector of the incident ray... 

Because of their inability to obtain fiber patterns. X-ray studies have not provided any conclusive evidence as to the nature of the conformations of syndiotactic poly(methacrylic acid) (PMMA). IR dichroism measurements cannot be applied because the oriented films cannot be prepared however, it was possible to obtain Raman depolarization data in aqueous solutions (i 7). Figure 5 shows Raman spectra in the solid state and in aqueous solution. Because of striking similarity of the spectra, it was concluded that the local conformations in both phases are similar. The frequency shift of two bands at 1452 and 1685 cm" was attributed to the differences in the extent of hydrogen bonding. On the basis of the IR and Raman activity of polarized and depolarized spectra, it was proposed that both solid and aqueous PMMA exist as helices that contain more than six monomer repeating units per two... 

In Fig. 16 we show the nematic order parameter 5n obtained from the type of IR dichroism measurements just described as a function of the deformation (L/Lq (mon)) of the same elastomer, where Lo(mon) is the length of the LSCE at Tn i. As one can see... 

IR-dichroism measurements of the CN-stretching mode revealed that the order parameter S extracted from the absorption coefficients along and perpendicular to the long axis of the molecules has a similar temperature dependence as in low molecular weight materials [16]. 

One of the practical problems in the case of IR dichroism measurements arises from the requirement of low band absorbance (roughly lower than 0.7 absorbance units) in order to permit use of the Beer-Lambert law. This condition implies use of very thin films in the range of 1 to 200 pm. NIR measurements alleviate this problem. 

Figure 28 Schematic diagram of IR dichroism measurement. An IR beam polarized in a direction parallel or perpendicular to the optical reference axis is used to probe the orientation of dipole transition moments.

The matrix IR spectra of la and several isotopomers (cU-la, l80-la) reveal details of the electronic structure of the carbene.23 In particular the red-shift of the C=0 stretching vibration (compared to p-benzoquinone) below 1500 cm-1 indicates a substantial contribution of the phenoxyl/phenyl resonance structure to the wave function of la. The C2V symmetry of the carbene was experimentally revealed by measuring the IR dichroism of partially oriented samples of matrix-isolated la. The orientation of la in an argon matrix was achieved by irradiation with linearly polarized light. 

Figure 19.1. Schematic diagram of a general pump-probe-detect laser spectrometer suitable for picosecond electronic absorption, infrared (IR) absorption, Raman, optical calorimetry, and dichroism measurements. For picosecond fluorescence—a pump-detect method, no probe pulse needs to be generated.

The absorption modes of (S)-3-phenyl-2-hydroxypropionic acid, (S)-3-phenyl-2-aminopropionic acid, and (S)-alanine adsorbed on a nickel plate or RNi were studied by Suetaka s group (71, 72). From the measurement of infrared (IR) dichroism in the reflection spectrum, the molecular orientation of the modifying reagent was deduced. Figures 19-21 show molecular orientations of (S)-2-hydroxy-3-phenylpropionic acid on a nickel plate and (R)-alanines on RNis modified at 5° and 100°C, respectively. 

We have done most of our studies on granular PTFE by compacting it into a preform and sintering to make a shape matching ASTM D-1708-66 for tensile testing. Orientation measurements by IR dichroism require thinner specimens, and for this purpose we used dispersion-grade material, commercially cast and processed into film (Dilectrix Corp., Farmingdale, NY). These films were used as received without further heat treatment. ... 

If two different three-dimensional arrangements in space of the atoms in a molecule are interconvertible merely by free rotation about bonds, they are called conformations if not, configurations.l85 Configurations represent isomers that can be separated, as previously discussed in this chapter. Conformations represent conformers, which are rapidly interconvertible and thus nonseparable. The terms conformational isomer and rotamer are sometimes used instead of conformer. A number of methods have been used to determine conformations.186 These include x-ray and electron diffraction, ir, Raman, uv, nmr,187 and microwave spectra,188 photoelectron spectroscopy,189 supersonic molecular jet spectroscopy,190 and optical rotatory dispersion and circular dichroism measurements.191 Some of these methods are useful only for solids. It must be kept in mind that the conformation of a molecule in the solid state is not necessarily the same as in solution.192 Conformations can be calculated by a method called molecular mechanics (p. 149). 

Another type of IR measurement which should be mentioned is the study of IR dichroism. Such data are valuable in determining gross molecular conformation and, more important, they contain information about electron distributions and mobilities as interesting as that obtainable from intensities. 

Matrix photolysis of several cyano-substituted phenyl azides with plane polarized light, combined with qualitative linear dichroism measurements in the IR spectra of the resulting matrices, has been used to deduce information about... 

A review of rheo-optical techniques by Sherman et al. (1996) notes that there has been an increase in the use of rheo-optic set-ups both for FT-IR dichroism and for dynamic IR dichroism spectroscopies for polymer melts and polymer blends. Skytt et al. (1996) highlight the use of simultaneous measurement of the transient or steady-state rheological properties and IR dichroism to characterize orientation in polymer melts. However, there is little reference to dual spectroscopic-rheological techniques for reactive polymer systems in the literature. 

While the IR dichroism and NMR studies can give information about the conformation at individual bonds, the small-angle neutron scattering (SANS) measures the global conformation of the polymers (Cotton et al., 1974). SANS has been successfully utilized, for example by Hardouin and coworkers (Hardouin et al, 1991 Leroux et al, 1993), to establish the evolution with the temperature of the global shape of some mesogen-jacketed liquid crystalline polymers from isotropic to the nematic phase. A pseudobinary system of fully protonated polymer (P4,4,4) with the same polymer... 

The strain-indueed orientation is analyzed with the help of birefringence measurements [107, 115, 120, 123] (realized in the isotropic state, close to the transition), measurements of the IR dichroism [124, 126], and X-ray experiments [93, 108,120, 122, 125]. It was observed that the orientational order parameter, labeled P2 on Fig. 32, obtained from X-ray experiments [122], quiekly inereases with strain at the beginning, then saturates at a P2 value of about 0.4-0.6 for a sample synthesized in an isotropic state [99, 107, 121, 122]. On condition that they were realized above the gel point [122], the networks display reproducible and reversible behavior independent of the sample history. 

Early measurements of the IR-dichroism in stretched samples of LCEs have been described [16]. The dependence of the IR di-chroism of the CN-stretching mode was used to obtain the absorption coefficients and to extract the order parameter as a function of temperature. However, while these measurements yield reliable results inside the nematic phase, the sensitivity of this method turns out to be not high enough to analyze quantitatively the influence of the applied stress on the order parameter and on... 

Wide-angle X-ray scattering (WAXS), SAXS, optical birefringence, IR dichroism and light scattering can be used for the detection and measurement of local and macroscopic orientation. One has mainly the following four cases of oriented crystalline polymer solid ... 

IR dichroism was used to study the orientational development in the high-speed spinning of Nylon 66 yarns. A relationship between IR dichroism and orientation factor was developed and a correlation between orientation measured by IR and X-ray diffraction demonstrated (372). 

A critical assessment was made of the use of ATR-IR spectroscopy for the study of the surface composition and orientation in uniaxially drawn PETP films. The use of reference bands commonly used in such analyses was examined, paying particular attention to the possible deleterious effects of sample orientation, incident polarisation and sample contact. The use of conventional sampling procedures to obtain in-plane surface dichroism measurements is shown. 25 refs. 

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